I. Field of the Invention
The present invention is an improved process and apparatus for cleaning and/or coating conductive metal surfaces using electro-plasma technology.
II. Background Art
Metals, ferrous and non-ferrous, usually need to be cleaned and/or protected from corrosion. As produced, steel normally has a film of mill oxide scale on its surface which is not uniformly adherent and renders the underlying material liable to galvanic corrosion. In most industrial applications this oxide scale must be removed and the underlying exposed metal coated against further oxidation, i.e. rust. Metals can also have other forms of contaminants, such as oils, chemical rust inhibitors, chemical polishing or buffing agents, grease, paint or simply dirt, which must be removed before industrial use.
Traditional methods of cleaning metals include acid pickling, which is environmentally unfriendly and requires huge volumes of fresh water to neutralize, grit or steel shot air and/or centrifugal blasting, salt or caustic bath cleaning, alkaline cleaning, mechanical brush cleaning or ultra high pressure water blasting. Bi-polar ultra-sonic cleaning will typically incorporate some chemical to enhance the process. Electrolytic cleaning involves the use of alkaline cleaning solutions and the polarity may be alternated. These electrolytic processes typically use low voltage (3 to 25 volts) with current densities of 1 to 20 Amps/dm2. In these processes, when the workpiece is the cathode, the surface may not only be cleaned but also “activated”, thereby giving any subsequent coating an improved adhesion. Electrolytic processes are not normally economically viable and in most cases cannot remove tenacious oxide scales.
Conventional electrolytic cleaning and plating processes operate in a low-voltage regime in which the electrical current increases monotonically with the applied voltage. In these conventional electrolytic processes, cleaning and/or plating, the generation of gas bubbles on the cathode do not, in any circumstance, experience the generation of plasma. In conventional electrolytic processes the increase in voltage produces many bubbles at the cathode surface, due to intensive electrolysis of the solution and liberation of Joule heat. At this stage the cell voltage-current characteristics conform to Ohm's Law. As voltage is increased, at some point the vapor inside the bubble is broken down and a spark discharge is observed. Continued voltage increase causes the spark intensity to increase. In this zone, known typically as an unstable region, the initiation of arc-discharge plasma is observed. Continued increase in voltage, above another point, a continuous glow occurs and remains as voltage increase continues, and in this region, current increases as voltage increases until a point is reached where electrical arcing occurs, which marks the termination of the plasma discharge region.
Prior patents such as UK-A-1399710, U.S. Pat. No. 5,958,604, U.S. Pat. No. 5,981,084, U.S. Pat. No. 5,700,366 and U.S. Pat. No. 6,585,875 in the field of electrical plasma processing (cleaning and coating) employ high voltage and operate in an electrical regime in which the current decreases or remains essentially the same as voltage is increased and are characterized by the formation of plasma at the onset of the unstable region.
In the current improved process the operating regime is substantially changed and has shifted out of the unstable region and operates in a region where as voltage is increased amperage also increases.
UK-A-1399710 teaches that the gas film must be continuous and the electrical regime which describes the current as decreasing or remaining fairly constant as voltage is increased described the “unstable regime” characterized as the descending half of the first current increase curve.
GB-A-1399710 teaches that a metal surface can be cleaned electrolytically without over-heating and without excessive energy consumption if the process is operated in a regime just beyond the unstable region, the “unstable region” being defined as one in which the current decreases with increasing voltage. By moving to higher voltages, where the current increases with increasing voltage a continuous film of gas/vapor is established over the treated surface, effective cleaning is obtained. Energy consumption is high (10 to 30 KWh/m2) as compared to the energy consumption required for acid pickling (0.4 to 1.8 KWh/m2).
SU-A-1599446 describes a high voltage electrolytic spark erosion cleaning process for welding rods which uses extremely high current densities, on the order of 1000/A/dm2, in a phosphoric acid solution.
SU-A-12244216 describes a micro-arc cleaning treatment for machine parts which operates at 100 to 350 volts using an anodic treatment. No particular method of electrolyte handling is taught.
JP-A-08003797 and DE-A-4031234 describes the use of high speed jets of electrolyte situated near the electrodes in the electrolytic cleaning baths to create high speed turbulent flow in the cleaning zone, but does not enter into the unstable region or utilize plasma creation.
EP-A-0037190 teaches the use of a single jet of electrolyte, without immersion of the workpiece, for cleaning radioactive contamination. The workpiece is anodic and the voltage is between 30 and 50 volts. Removal of the oxide is held to be undesirable.
EP-A-0406417 describes a continuous process for drawing copper wire from copper rod in which the rod is plasma cleaned before the drawing operation. The “plasmatron” housing is the anode and the wire is also surrounded by an inner co-axial anode in the form of a perforated U-shaped sleeve. In order to initiate plasma production the voltage is maintained at a low but unspecified value, the electrolyte level above the immersed wire is lowered, and the flow-rate decreased in order to stimulate the onset of a discharge at the wire surface. Low-voltage electrolytic cleaning is widely used to prepare metal surfaces for electro-plating or other coating treatments, these processes cannot remove mill oxide scales without unacceptably high energy consumption.
Such electrolytic cleaning processes must normally be used in conjunction with other types of cleaning processes, in a multi-stage operation.
WO-A-97/35052 describes an electrolytic process for cleaning electrically conducting surfaces suing an electro-plasma (arc-discharge) in which a liquid electrolyte flows through one or more holes in a anode held at high DC voltage and impinges on the workpiece (cathode) thus providing and electrically conductive path. The system is operated in a regime in which the electrical current decreases or remains substantially constant with increase in the voltage applied between the anode and the cathode and in a regime in which discrete bubbles of gas and or vapor are present on the surface of the workpiece during treatment.
WO-A-97/35051 describes an electrolytic process for cleaning and coating electrically conducting surfaces which is similar to the process described in WO-A-97/35052 except that the anode comprises a metal for metal coating of the surface of the workpiece.
In WO-A-97/35051 & 35052 an arc discharge or electro-plasma is formed on the surface of the workpiece and is established within the bubble layer. The plasma has the effect of rapidly removing mill-scale and other contaminants from the surface of the workpiece, leaving a clean metal surface which may also be passivated (resistance to corrosion). If the anode is constructed from a non-inert materials, such as a non-refractory metal, then metal atoms are transferred from the anode to the cathode, providing a metal coating on the cleaned surface.
Coating may also be achieved under the regime of operation described above by using an inert anode and an electrolyte containing ions of the metal to be coated as described in WO-A-99/15714. In this case the process becomes a special form of electroplating, but because it occurs at high voltage in the presence of an arc discharge the plating is faster than normal electroplating and the coating has better adhesion to the substrate metal.
U.S. Pat. No. 4,360,410 Fletcher et al. describes the use of foam for an improved electroplating process. This is a typical electroplating process where low voltage is utilized for ion transfer, without arc discharge or plasma generation. Fletcher et al operates in a different electrical regime which is typical of conventional electrolytic processes. The importance of Fletcher et al is the verification that foam improves uniformity.
WO-A-98/32892 describes a process which operates essentially in the manner described in WO-A-99/15714 but uses a conductive gas/vapor mixture as the conductive medium. This gas/vapor mixture is generated within a two or multi-chambered area before being ejected into the working gap through holes in the anode plate. The gas/vapor mixture is generated by heating an aqueous electrolyte within the anode chambers (chambers adjacent to the anode plate, above or below the anode plate itself) to the boiling point or above, and the anode chambers may be heated either by the main electric current or by independent electrical heaters.
WO-01/09410 A1-U.S. Pat. No. 6,585,875-describes a process similar to WO-A-98/32892 and WO-A-99/15714 and claims an improved process in which, an electro-plasma (arc-discharge) is employed to clean and/or apply a metal coating to an electrically conductive surface. For example steel, in which the electrically conductive pathway is provided by a foaming electrolyte which fills the space between the anode and the cathode and provides advantages with respect to lower power consumption, more uniform surface treatment and greater latitude in the size of the gap between the anode and the workpiece.
U.S. Pat. No. 6,585,875 teaches an improved process in which arc-discharge electro-plasma is employed to clean and/or apply a metal coating to an electrically conductive surface, in which, the electrically conductive pathway is provided by a foaming electrolyte which fills the space between the anode and the cathode and provides advantages with respect to lower power consumption, more uniform surface treatment and greater latitude in the size of the gap between the anode and the cathode.
The present process has changed the creation (foam & plasma) of the process substantially both from the mechanical aspect, by creating a very simple device to utilize the process, and changes the internal mechanism for electrolyte delivery. It also changes the mechanics of foam creation, enhances ionization and causes the use of turbulence to enhance foam creation, foam density, increases plasma evolution, removes debris from the work chamber, and stabilizes the plasma by utilizing a denser foam which stabilizes the gas envelope, which forms around the workpiece. The changes create a more user-friendly apparatus, which allows for greater commercialization by creating the ability to process multiple work-pieces inline without the need for specialized machine parts and reduces or eliminates wear parts. The new process, which is an improved embodiment of rapid advancements in the field of electro-plasma processing also creates a multi-functional process which has the potential to reduce or eliminate the need for environmentally unfriendly chemicals, such as zinc-phosphate to be used as lubricant carriers in the metals industry. The advancements also create the ability to use heretofore waste materials to become useful, commercial products.